U.S. patent number 7,016,570 [Application Number 10/692,805] was granted by the patent office on 2006-03-21 for optical signal processor.
This patent grant is currently assigned to Sumitomo Electric Industries, Ltd.. Invention is credited to Tomomi Sano, Michiko Takushima.
United States Patent |
7,016,570 |
Takushima , et al. |
March 21, 2006 |
Optical signal processor
Abstract
An optical signal processor comprises fiber collimators, a first
diffraction grating device, a second diffraction grating device, a
first half-wave plate, and a second half-wave plate. Each of the
diffraction grating devices is of reflection type having a
diffracting surface parallel to a yz plane and a grating direction
parallel to a z axis. The diffraction grating device diffracts the
light outputted from the fiber collimator after collimation. The
diffraction grating device diffracts the light diffracted by the
diffraction grating device. The half-wave plates having respective
optic axes in directions different from each other by 45 degrees
are bonded together and are disposed on an optical path between the
diffraction grating devices.
Inventors: |
Takushima; Michiko (Yokohama,
JP), Sano; Tomomi (Yokohama, JP) |
Assignee: |
Sumitomo Electric Industries,
Ltd. (Osaka, JP)
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Family
ID: |
27764325 |
Appl.
No.: |
10/692,805 |
Filed: |
October 27, 2003 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20040136654 A1 |
Jul 15, 2004 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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PCT/JP03/01588 |
Feb 14, 2003 |
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Foreign Application Priority Data
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Feb 27, 2002 [JP] |
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P2002-052011 |
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Current U.S.
Class: |
385/37 |
Current CPC
Class: |
G02B
6/2931 (20130101); G02B 27/4261 (20130101); G01J
3/18 (20130101); G02B 6/2938 (20130101); G06E
3/00 (20130101); G02B 6/29308 (20130101) |
Current International
Class: |
G02B
6/34 (20060101) |
Field of
Search: |
;385/31,37,11 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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59-60408 |
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Apr 1984 |
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JP |
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59-60408 |
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Apr 1984 |
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JP |
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02-159528 |
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Jun 1990 |
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JP |
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2-159528 |
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Jun 1990 |
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JP |
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05-100114 |
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Apr 1993 |
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JP |
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5-100114 |
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Apr 1993 |
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JP |
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05-215918 |
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Aug 1993 |
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JP |
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5-215918 |
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Aug 1993 |
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JP |
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07-151982 |
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Jun 1995 |
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JP |
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7-151982 |
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Jun 1995 |
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JP |
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2001-4447 |
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Jan 2001 |
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JP |
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P2001-4447 |
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Jan 2001 |
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JP |
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2002-323374 |
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Nov 2002 |
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JP |
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P2002-323374 |
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Nov 2002 |
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JP |
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Primary Examiner: Song; Sarah
Attorney, Agent or Firm: McDermott Will & Emery LLP
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATION
This is a Continuation-In-Part application of International Patent
application serial No. PCT/JP03/01588 filed on Feb. 14, 2003, now
pending.
Claims
What is claimed is:
1. An optical signal processor comprising: a first diffraction
grating device for diffracting incident light, the first
diffraction grating having a diffracting surface arranged on a
first reference plane; a second diffraction grating device for
diffracting the light diffracted by the first diffraction grating
device, the second diffraction grating device having a diffracting
surface arranged on a second reference plane apart from and
parallel to the first reference plane; and first and second
half-wave plates, disposed on an optical path between the first and
second diffraction grating devices, having their respective optical
axes disposed at an angle of 45 degrees from each other.
2. An optical signal processor according to claim 1, wherein the
first and second half-wave plates are arranged orthogonal to an
optical axis of light having a center wavelength in a wavelength
band in use.
3. An optical signal processor according to claim 1, wherein the
first and second diffraction grating devices have the same grating
direction.
4. An optical signal processor according to claim 1, wherein each
of the first and second diffraction grating devices is a reflection
type diffraction grating device.
5. An optical signal processor according to claim 1, wherein each
of the first and second diffraction grating devices is a
transmission type diffraction grating device.
6. An optical signal comprising: a first diffraction grating device
for diffracting incident light, the first diffraction grating
device having a diffracting surface arranged on a first reference
plane; a second diffraction grating device for diffracting the
light diffracted by the first diffraction grating device, the
second diffraction grating device having a diffracting surface
arranged on the first reference plane; a mirror disposed on an
optical path between the first and second diffraction grating
devices, the mirror having a reflecting surface arranged on a
second reference plane apart from and parallel to the first
reference plane; and first and second half-wave plates, disposed on
an optical path between the first diffraction grating device and
the mirror, or on an optical path between the second diffraction
grating device and the mirror, having their respective optical axes
disposed at an angle of 45 degrees from each other.
7. An optical signal processor according to claim 6, wherein the
first and second diffraction grating devices are integrated with
each other.
8. An optical signal processor according to claim 6, wherein the
first and second diffraction grating devices have the same grating
direction.
9. An optical signal processor according to claim 6, wherein each
of the first and second diffraction grating devices is a reflection
type diffraction grating device.
10. An optical signal processor according to claim 6, wherein each
of the first and second diffraction grating devices is a
transmission type diffraction grating device.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical signal processor
including a diffraction grating device.
2. Related Background Art
As an optical signal processor including a diffraction grating
device, one disclosed in Japanese Patent Application Laid-Open No.
2001-4447 has been known for example. The optical signal processor
disclosed in the publication intends to demultiplex light with an
excellent wavelength resolution by causing a diffraction grating
device to diffract the light twice. By propagating light in the
direction opposite from that at the time of demultiplexing, the
optical signal processor can also multiplex the inputted light
having a certain wavelength region and output thus multiplexed
light.
Meanwhile, the state of polarization of light varies as
characteristics of the optical transmission line fluctuate upon
changes in environments and the like, whereby the light to be fed
into and processed by the optical signal processor does not always
have a constant state of polarization. Therefore, it is desirable
for the processing of light in the optical signal processor to be
less dependent on the state of polarization of input light.
However, the efficiency of light diffraction in diffraction grating
devices has been known to vary depending on the polarization
direction of incident light.
For overcoming such a problem, the optical signal processor
disclosed in the above-mentioned publication disposes a half-wave
plate on the optical path between the first and second diffracting
actions caused by the diffraction grating device. The half-wave
plate has an optic axis in a direction parallel to a plane
perpendicular to the optical axis of incident light. When linearly
polarized light having a polarization direction inclined by an
angle .theta. with respect to the optic axis is incident on the
half-wave plate, the polarization direction of the light is rotated
by an angle 2.theta., and the light is emitted as linearly
polarized light having a polarization direction inclined by an
angle (-.theta.) with respect to the optic axis. If the angle
.theta. is 45 degrees, the half-wave plate can output the incident
linearly polarized light as linearly polarized light having a
direction orthogonal thereto. By utilizing such an effect of the
half-wave plate, the optical signal processor disclosed in the
above-mentioned publication rotates the polarization direction of
light by 90 degrees between the first and second diffracting
actions caused by the diffraction grating device, thereby lowering
the dependence on polarization.
SUMMARY OF THE INVENTION
The inventor studied the conventional technique mentioned above
and, as a result, has found the following problem. Namely, for
sufficiently lowering the dependence on polarization, it is
necessary for the optical signal processor disclosed in the
above-mentioned publication to set the direction of the optic axis
of the half-wave plate strictly. The direction of the optic axis of
the half-wave plate is adjusted while monitoring input/output
characteristics of the optical signal processor. On the other hand,
the optical signal processor is required to be as small as
possible. Therefore, the direction of the optic axis of the
half-wave plate arranged in a narrow space within the optical
signal processor is hard to adjust accurately. Hence, it is hard to
realize a small-size optical signal processor whose dependence on
polarization is lowered.
For overcoming the above-mentioned problem, it is an object of the
present invention to provide an optical signal processor which can
easily lower the dependence on polarization even when it has a
small size.
The optical signal processor in accordance with the present
invention comprises (1) a first diffraction grating device for
diffracting light inputted; (2) a second diffraction grating device
for diffracting the light diffracted by the first diffraction
grating device; and (3) first and second half-wave plates, disposed
on an optical path between the first and second diffraction grating
devices, having respective optic axes in directions different from
each other by 45 degrees.
The light fed into the optical signal processor is diffracted by
the first diffraction grating device and then, with its
polarization direction rotated by the first and second half-wave
plates, is diffracted again by the second diffraction grating
device. The first and second half-wave plates have respective optic
axis directions different from each other by 45 degrees, and can
rotate the polarization direction of incident light by 90 degrees
regardless of their individual optic axis directions and then emit
thus rotated light.
Here, the first and second diffraction grating devices are
preferably arranged parallel to each other. As a consequence, the
respective optical axes of individual signal light components
diffracted by the second diffraction grating device become parallel
to each other, which facilitates subsequent signal processing
operations such as connection to a fiber.
Preferably, the first and second half-wave plates are arranged
orthogonal to an optical axis of light having a center wavelength
in a wavelength band in use. This optimizes the lowering of
dependence on polarization in the wavelength band in use.
Preferably, a mirror is disposed on the optical path between the
first and second diffraction grating devices, whereas the first and
second half-wave plates are disposed on an optical path between the
first diffraction grating device and the mirror, or on an optical
path between the second diffraction grating device and the mirror.
As a consequence, the light fed into the optical signal processor
is diffracted by the first diffraction grating device and then,
with its polarization direction rotated by the first and second
half-wave plates, is reflected by the mirror. Thus reflected light
is diffracted again by the second diffraction grating device.
Alternatively, the light fed into the optical signal processor is
diffracted by the first diffraction grating device, and then is
reflected by the mirror. Thereafter, with its polarization
direction rotated by the first and second half-wave plates, the
reflected light is diffracted again by the second diffraction
grating device.
Here, the mirror is preferably arranged parallel to the first or
second diffraction grating device. As a consequence, the respective
optical axes of individual signal light components diffracted by
the second diffraction grating device become parallel, which
facilitates subsequent signal processing operations such as
connection to a fiber.
Preferably, the first and second diffraction grating devices are
integrated with each other. This facilitates adjustment at the time
of assembling.
Preferably, the first and second diffraction grating devices have
the same grating direction. This enhances diffraction
efficiency.
The present invention will become more fully understood from the
detailed description given hereinbelow and the accompanying
drawings. They are given by way of illustration only, and thus
should not be considered limitative of the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of the optical signal processor in
accordance with a first embodiment;
FIGS. 2A and 2B are explanatory views of a method of adjusting the
optic axes of half-wave plates in the optical signal processor in
accordance with the first embodiment;
FIG. 3 is a chart showing demultiplexing characteristics of the
optical signal processor in accordance with an example;
FIG. 4 is a chart showing polarization-dependent loss of the
optical signal processor in accordance with the example;
FIG. 5 is a chart showing the relationship between the
polarization-dependent loss and the optic axis direction of a
half-wave plate in the optical signal processor in accordance with
the example; and
FIG. 6 is a schematic diagram of the optical signal processor in
accordance with a second embodiment.
FIGS. 7A and 7B are views showing the case where grating directions
are same between two diffraction grating devices.
FIGS. 8A and 8B are views showing the case where grating directions
are different between two diffraction grating devices.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the following, embodiments of the present invention will be
explained in detail with reference to the accompanying drawings. In
the explanation of the drawings, constituents identical to each
other will be referred to with numerals identical to each other
without repeating their overlapping descriptions.
First, the principle of the present invention will be explained. It
is assumed in an xyz orthogonal coordinate system that a wave plate
is disposed parallel to the xy plane and that light advances
parallel to the z axis. It is also assumed that the wave plate has
an optic axis, located on a plane parallel to the xy plane, in a
direction forming an angle .theta. with respect to the y axis. Let
E.sub.0x be the polarized light component in the x-axis direction
of light advancing in the z-axis direction so as to be made
incident on the wave plate, and E.sub.0y be the polarized light
component in the y-axis direction thereof. Let E.sub.1x be the
polarized light component in the x-axis direction of the light
emitted from the wave plate, and E.sub.1y be the polarized light
component in the y-axis direction thereof.
Then, the relational expression of
.times..times..function..theta..phi..times..times..times.
##EQU00001## holds between the incident light (E.sub.0x, E.sub.0x)
and the emitted light (E.sub.1x, E.sub.1x). Here, the matrix
J(.theta., .phi.) is a Jones matrix indicating input/output
characteristics of light in the wave plate, and is represented by
the expression of
.function..theta..phi..times..times..theta..times..times..theta..times..t-
imes..theta..times..times..theta..times.e.times..phi.e.times..phi..times..-
times..times..theta..times..times..theta..times..times..theta..times..time-
s..theta. ##EQU00002## Here, j is an imaginary unit. .phi. is a
phase difference occurring when light having respective polarized
light components in x and y directions passes through the wave
plate. .phi.=.pi. in the case of a half-wave plate.
In the optical signal processor disclosed in the publication listed
in the background art section, a half-wave plate in which
.phi.=.pi./4 and .phi.=.pi. is used. The Jones matrix J(.pi./4,
.pi.) of this half-wave plate is represented by the expression of
.function..pi..pi. ##EQU00003## This expression means that the
half-wave plate can rotate the polarization direction of incident
light by 90 degrees and emit thus rotated light.
In general, the Jones matrix of the half-wave plate is given when
.phi.=.pi. in the above-mentioned expression, so as to be
represented by the expression of
.function..theta..pi..function..times..theta..times..theta..times..times.-
.times..theta..theta..times..times..times..theta..theta..function..times..-
theta..times..theta. ##EQU00004## As can be seen from this
expression, the Jones matrix of the half-wave plate depends on the
angle .theta.. Hence, for sufficiently lowering the dependence on
polarization, the optical signal processor disclosed in the
publication listed in the background art section is required to set
the direction of the optic axis of the half-wave plate
strictly.
Therefore, the present invention uses two half-wave plates in
combination. The optic axis of the first half-wave plate is assumed
to form the angle .theta. with respect to the y axis. The optic
axis of the second half-wave plate is assumed to form the angle
(.theta.+.pi./4) with respect to the y axis. The Jones matrix of
the first half-wave plate is given by the above-mentioned
expression (4), whereas the Jones matrix of the second half-wave
plate is represented by the expression of
.function..theta..pi..pi..times..times..times..theta..theta..function..ti-
mes..theta..times..theta..function..times..theta..times..theta..times..tim-
es..times..theta..theta. ##EQU00005##
The total Jones matrix of the first and second half-wave plates is
given by the product of the above-mentioned expressions (4) and
(5), and is represented by the expression of
.function..theta..pi..pi..function..theta..pi. ##EQU00006## As can
be seen from this expression, the combination of the first and
second half-wave plates can rotate the polarization direction of
incident light by 90 degrees and emit thus rotated light
independently of the .theta. value. Namely, the combination of the
first and second half-wave plates having respective optic axes in
directions different from each other by 45 degrees can simply
rotate the polarization direction of incident light by 90 degrees
and output thus rotated light.
The optical signal processor in accordance with the present
invention is based on the foregoing principle, and uses two
half-wave plates in combination.
First Embodiment
A first embodiment of the optical signal processor in accordance
with the present invention will now be explained. FIG. 1 is a
schematic diagram of the optical signal processor 1 in accordance
with the first embodiment. The optical signal processor 1 shown in
this diagram comprises fiber collimators 110 to 113, a first
diffraction grating device 121, a second diffraction grating device
122, a first half-wave plate 131, and a second half-wave plate 132.
For convenience of explanation, an xyz orthogonal coordinate system
is also shown in this diagram. Light is assumed to advance in
parallel with the xy plane in the optical signal processor 1.
Each of the fiber collimators 110 to 113 comprises an optical fiber
with a spherically processed leading end part or an optical fiber
with a leading end connected to a lens, and has a collimating
function. Each of the fiber collimators 110 to 113 can collimate
the light having arrived at the leading end of the optical fiber
after propagating therethrough and output thus collimated light, or
converge the light having arrived at the leading end from the
outside and propagate thus converged light through the optical
fiber.
The diffraction grating device 121 is of reflection type, and has a
diffracting surface parallel to the yz plane and a grating
direction parallel to the z axis. When the light outputted from the
collimator 110 after collimation is incident, the diffraction
grating device 121 diffracts the light at a diffraction angle
corresponding to the wavelength. Letting d be the grating constant
of the diffraction grating device 121, .beta..sub.1 be the incident
angle of light, and .beta..sub.2 be the diffraction angle of light
having a wavelength .lamda., the relationship of m.lamda.=d(sin
.beta..sub.1+sin .beta..sub.2) (7) holds among these angles. Here,
m is the order of diffraction.
The diffraction grating device 122 is of reflection type having a
diffracting surface parallel to the yz plane and a grating
direction parallel to the z axis. The diffraction grating devices
121, 122 have respective diffracting surfaces opposing each other.
The diffraction grating device 122 has the same grating constant d
as with the diffraction grating device 121. The diffraction grating
device 122 diffracts the light diffracted by the diffraction
grating device 121. Here, the incident angle of light having the
wavelength .lamda. onto the diffraction grating device 122 is
.beta..sub.2, whereas the diffraction angle of light in the
diffraction grating device 122 is .beta..sub.1 regardless of the
wavelength .lamda.. Namely, the individual wavelength light
components diffracted by the diffraction grating device 122 advance
in parallel with each other.
The fiber collimator 111 converges and inputs the light having a
wavelength .lamda..sub.1 diffracted by the diffraction grating
device 122. The fiber collimator 112 converges and inputs the light
having a wavelength .lamda..sub.2 diffracted by the diffraction
grating device 122. The fiber collimator 113 converges and inputs
the light having a wavelength .lamda..sub.3 diffracted by the
diffraction grating device 122. When light advances as in the
foregoing, the optical signal processor 1 is used as an optical
demultiplexer which inputs light outputted from the fiber
collimator 110, demultiplexes thus inputted light, and outputs thus
demultiplexed individual wavelength light components into any of
the fiber collimators 111 to 113. When light advances in the
opposite direction, the optical signal processor 1 is used as an
optical multiplexer which inputs respective wavelength light
components outputted from the fiber collimators 111 to 113,
multiplexes thus inputted light components, and outputs thus
multiplexed light to the fiber collimator 110.
The two half-wave plates 131, 132, which have respective optic axes
in directions different from each other by 45 degrees, are bonded
together and disposed on the optical path between the diffraction
grating devices 121 and 122. The combination of the two half-wave
plates 131, 132 has a total Jones matrix represented by the
above-mentioned expression (6), and can rotate the polarization
direction of incident light by 90 degrees and emit thus rotated
light.
Therefore, if the two half-wave plates 131, 132 are set so that
their optic axes are in respective directions different from each
other by 45 degrees, the small-size optical signal processor 1 can
easily be assembled by using them. At the time of assembling, the
respective optic axes of the half-wave plates 131, 132 may be in
any directions, whereby the optical signal processor 1 can easily
lower its dependence on polarization even when it has a small
size.
The diffraction grating devices 121, 122 are arranged parallel to
each other. The half-wave plates 131, 132 are arranged orthogonal
to the optical axis of the center wavelength of the wavelength band
(.lamda..sub.1 to .lamda..sub.3) of light to be processed by the
optical signal processor 1. Such an arrangement can lower the
dependence of the optical signal processor 1 on polarization more
fully.
FIGS. 2A and 2B are explanatory views of a method of adjusting the
optic axes of the half-wave plates 131, 132 in the optical signal
processor 1 in accordance with the first embodiment. As shown in
FIG. 2A, a light source 910, a power meter 920, and polarizers 931,
932 are prepared. The light source 910 and the optical power meter
920 are arranged such that the light outputted from the former can
be received by the latter, whereas the polarizers 931, 932 are
disposed on the optical path between the light source 910 and the
power meter 920. Then, one of the polarizers 931, 932 is rotated
about the optical axis, so as to be adjusted such that the light
received by the power meter 920 attains the maximum power. As a
consequence, the respective optic axis directions of the polarizers
931, 932 are set parallel to each other. The arrows in the
polarizers in the drawing indicate the optic axis directions of the
polarizers.
Subsequently, as shown in FIG. 2B, the half-wave plates 131, 132
are disposed on the optical path between the polarizers 931, 932.
Then, one of the half-wave plates 131, 132 is rotated about the
optical axis, so as to be adjusted such that the light received by
the power meter 920 attains the minimum power. As a consequence,
the half-wave plates 131, 132 are set so as to have respective
optic axis directions different from each other by 45 degrees.
The half-wave plates 131, 132 having set their respective optic
axis directions as such are bonded together, and the resulting
product is used for assembling the optical signal processor 1,
whereby the small-size optical signal processor 1 having reduced
the dependence on polarization can be assembled easily. If the
optical system shown in FIG. 2A is prepared beforehand, the optic
axes of the half-wave plates 131, 132 can be adjusted in a step
different from the step of assembling the optical signal processor
1, which is excellent in terms of mass productivity.
A specific example of the optical signal processor 1 in accordance
with the first embodiment will now be explained. In this example,
each of the diffraction grating devices 121, 122 has a grating
constant of 1.7 .mu.m, whereas wavelengths of light to be processed
are 1530 nm, 1550 nm, and 1570 nm. The incident angle .beta..sub.1
of light from the fiber collimator 110 onto the diffraction grating
device 121 is 15 degrees, whereas the diffraction angle
.beta..sub.2 of light having a center wavelength of 1550 nm in the
first diffraction grating device 121 is 40.8 degrees. Each of the
half-wave plates 131, 132 is arranged orthogonal to the optical
axis of the light having the center wavelength of 1550 nm from the
first diffraction grating device 121 to the second diffraction
grating device 122. FIGS. 3 to 5 show various characteristics of
the optical signal processor in accordance with the example.
FIG. 3 is a chart showing demultiplexing characteristics of the
optical signal processor in accordance with the example. This chart
shows the transmission characteristic of light from the fiber
collimator 110 to the fiber collimator 111, the transmission
characteristic of light from the fiber collimator 110 to the fiber
collimator 112, and the transmission characteristic of light from
the fiber collimator 110 to the fiber collimator 113. As shown in
this chart, the light emitted from the fiber collimator 110 to the
diffraction grating device 121 is diffracted by the diffraction
grating devices 121, 122 at respective diffraction angles
corresponding to wavelengths, so as to be demultiplexed. Then,
light components having wavelengths of 1530 nm, 1550 nm, and 1570
nm are made incident on the fiber collimators 111, 112, and 113,
respectively.
FIG. 4 is a chart showing polarization-dependent loss (PDL) of the
optical signal processor in accordance with the example (indicated
by L1). This chart also shows polarization-dependent loss of a
comparative example without the half-wave plates 131, 132
(indicated by L2). In each of the example and comparative example,
the polarization-dependent loss of light from the fiber collimator
110 to the fiber collimator 112 is shown. As can be seen from this
chart, since the optical signal processor in accordance with the
example is provided with two half-wave plates 131, 132 having
respective optic axis directions different from each other by 45
degrees, its polarization-dependent loss is improved by about 0.8
dB over the comparative example.
FIG. 5 is a chart showing the relationship between the
polarization-dependent loss and the optic axis direction of a
half-wave plate in the optical signal processor in accordance with
the example (indicated by L1). This chart also shows the case of a
comparative example (indicated by L2) in which only one half-wave
plate is disposed between the diffraction grating devices 121, 122.
In each of the example and comparative example, the
polarization-dependent loss of light having a wavelength of 1550 nm
from the fiber collimator 110 to the fiber collimator 112 is shown
with respect to the angle of rotation of the half-wave plate about
the optical axis. As can be seen from this chart, the range of
fluctuation in polarization-dependent loss when rotating the
half-wave plate about the optical axis is very small in the
example, while it is about 0.8 dB in the comparative example. Thus,
since the half-wave plates 131, 132 set with respective optic axis
directions different from each other by 45 degrees are used, this
embodiment can make it easy to assemble the optical signal
processor 1 having a small size with reduced polarization-dependent
loss.
Second Embodiment
A second embodiment of the optical signal processor in accordance
with the present invention will now be explained. FIG. 6 is a
schematic diagram of the optical signal processor 2 in accordance
with the second embodiment. The optical signal processor 2 shown in
this diagram comprises fiber collimators 210 to 213, a diffraction
grating device 220, a first half-wave plate 231, a second half-wave
plate 232, and a mirror 240. For convenience of explanation, this
diagram also shows an xyz orthogonal coordinate system. It is
assumed in the optical signal processor 2 that light advances in
parallel with the xy plane.
Each of the fiber collimators 210 to 213 comprises an optical fiber
with its leading end processed into a spherical form, so as to
exhibit a collimator function. Each fiber collimator can collimate
the light having arrived at the leading end of the optical fiber
after propagating therethrough and output thus collimated light, or
converge the light having arrived at the leading end from the
outside and propagate thus converged light through the optical
fiber.
The diffraction grating device 220 is of reflection type, and has a
diffracting surface parallel to the yz plane and a grating
direction parallel to the z axis. When the light outputted from the
fiber collimator 210 after collimation is incident, the diffraction
grating device 220 diffracts the light at a diffraction angle
corresponding to the wavelength. When the light reflected by the
mirror 240 is incident, the diffraction grating device 220
diffracts the light as well. Namely, the diffraction grating device
220 is one in which the first and second diffraction grating
devices are integrated with each other.
The reflecting surface of the mirror 240 opposes the diffracting
surface of the diffraction grating device 220 and is parallel to
the diffraction grating device 220. The light diffracted by the
diffraction grating device 220 is made incident on and reflected by
the mirror 240, so as to be made incident on the diffraction
grating device 220 again.
Letting d be the grating constant of the diffraction grating device
220, .beta..sub.1 be the incident angle of light from the fiber
collimator 210 onto the diffraction grating device 220, and
.beta..sub.2 be the diffraction angle of light having a wavelength
.lamda. at this time, the above-mentioned expression (7) holds
among these angles. Also, the incident angle of light from the
mirror 240 onto the diffraction grating device 220 is .beta..sub.2,
whereas the diffraction angle of light at this time is .beta..sub.1
regardless of the wavelength .lamda.. Namely, the individual
wavelength light components diffracted twice by the diffraction
grating device 220 advance in parallel with each other.
The fiber collimator 211 converges and inputs light having a
wavelength .lamda..sub.1 diffracted twice by the diffraction
grating device 220. The fiber collimator 212 converges and inputs
light having a wavelength .lamda..sub.2 diffracted twice by the
diffraction grating device 220. The fiber collimator 213 converges
and inputs light having a wavelength .lamda..sub.3 diffracted twice
by the diffraction grating device 220. When light advances as in
the foregoing, the optical signal processor 2 is used as an optical
demultiplexer which inputs light outputted from the fiber
collimator 210, demultiplexes thus inputted light, and outputs thus
demultiplexed individual signal light components into any of the
fiber collimators 211 to 213. When light advances in the opposite
direction, the optical signal processor 2 is used as an optical
multiplexer which inputs individual wavelength light components
outputted from the fiber collimators 211 to 213, multiplexes thus
inputted light components, and outputs thus multiplexed light to
the fiber collimator 210.
The two half-wave plates 231, 232, which have respective optic axes
in directions different from each other by 45 degrees, are bonded
together and are disposed on the optical path between the
diffraction grating device 220 and the mirror 240. The combination
of the two half-wave plates 231, 232 has a total Jones matrix
represented by the above-mentioned expression (6), and can rotate
the polarization direction of incident light by 90 degrees and emit
thus rotated light. The half-wave plates 231, 232 may be disposed
on the optical path from the first diffracting action by the
diffraction grating device 220 to the reflection by the mirror 240,
or on the optical path from the reflection by the mirror 240 to the
second diffracting action by the diffraction grating device
220.
Therefore, if the two half-wave plates 231, 232 are set so as to
have respective optic axis directions different from each other by
45 degrees, the small-size optical signal processor 2 can be
assembled easily by using them. At the time of assembling, the
respective optic axes of the half-wave plates 231, 232 may be in
any directions, whereby the optical signal processor 2 can easily
lower its dependence on polarization even when it has a small
size.
The half-wave plates 231, 232 are arranged parallel to each other.
Further, each of the half-wave plates 231, 232 is arranged
orthogonal to the optical axis of the center wavelength of the
wavelength band (.lamda..sub.1 to .lamda..sub.3) of light to be
processed by the optical signal processor 2. Such an arrangement
can lower the dependence of the optical signal processor 2 on
polarization more fully.
Without being restricted by the above-mentioned embodiments, the
present invention can be modified in various manners. For example,
though each of the optical signal processor in accordance with the
above-mentioned embodiments comprises a reflection type diffraction
grating device, they may be provided with transmission type
diffraction grating devices.
Though the respective diffracting surfaces of the diffraction
grating devices 121, 122 are parallel to each other in the first
embodiment, the diffracting surfaces may not be parallel to each
other. Though the diffracting surface of the diffraction grating
device 220 and the reflecting surface of the mirror 240 are
parallel to each other in the second embodiment, these surfaces may
not be parallel to each other. When they are not parallel to each
other, the wavelength gap can be expanded.
Though the grating directions of the gratings 123, 124 provided on
diffracting surfaces 121a, 122a are same between diffraction
grating devices 121, 122 and both parallel to the z axis as shown
in FIGS. 7A and 7B, the grating directions of the gratings 123, 124
may be different from each other as shown in FIGS. 8A and 8B.
However, the first and second diffraction grating devices
preferably have the same grating direction. This enhances
diffraction efficiency.
As is described in detail, the present invention can easily
assemble a small-size optical signal processor by using two
half-wave plates. At the time of assembling, the respective optic
axes of two half-wave plates may be in any directions, whereby the
optical signal processor can easily lower its dependence on
polarization even when it has a small size.
It is apparent from the above description of the present invention
that the present invention can be modified in various ways. Such
modifications are embraced in the present invention without
departing from the spirit and scope of the present invention and
all improvements obvious to those skilled in the art are included
in the scope of the claims which follow.
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